This is a weird hypothesis that I thought up when I was renaming some paper pdfs I've collected. It was inspired by a speculation I've seen that vertebrate side fins / limbs are ectopic tails; this was inspired by how limbs express Hox genes that are also expressed in tails. Ectopic tails may be able to form more easily because vertebrate tails extend past the anus, which is to the rear of most of the internal organs. Compare the situation of arthropods, annelids, and others, where the anus is at the absolute rear of the body, with no tail extending past it. Post-anal tails are shared by other chordates and by hemichordates. Which leaves echinoderms the remaining group of deuterostomes; the aforementioned terminal-anus animals are protostomes. How chordates, hemichordates, and echinoderms are related has been revised as a result of molecular evidence. An obvious overall-feature tree is: ((chordates, hemichordates), echinoderms) But molecular evidence suggests: (chordates, (hemichordates, echinoderms)) Which suggests that post-anal tails are an ancestral feature of deuterostomes, meaning that echinoderms have had an ancestor with them. But where might we find such a feature in living ones? Consider starfish and brittle-star anatomy. A starfish has a central disc with most of the internal organs, and with five arms radiating outwards. The mouth is on the bottom of the disc, the anus on the top. And those arms could possibly have originally been extra tails; their anatomy fits, since like chordate and hemichordate tails, they have only a limited set of internal organs. The main problem with the five-tail hypothesis is how the other echinoderms fit in; they also have radial symmetry, but they do not have comparable limbs.